uboot-imx

dtoc: Make use of the new pylibfdt methods

Now that pylibfdt supports a fuller API we don't need to directly call
the libfdt stubs. Update the code to use the Fdt methods instead.

Some other cases remain which will be tidied up in a later commit, since
they need larger changes.

Signed-off-by: Simon Glass <sjg@chromium.org>